Freight trucks travelling on highways have been well known vehicles for many years, the larger of such trucks generally having the form of a tractor pulling a semitrailer and being connected thereto by the combination of a horizontal fifth wheel plate carried by the tractor and a king pin carried on the forward portion of the semitrailer. This truck arrangement, of course, permits a pivoting action about the fifth wheel to provide a facility for maneuvering the truck into small loading zones and a much shorter turning radius for the entire truck.
In more recent years the trucking industry, in seeking ways to carry more freight on each trip has experimented with the addition of a trailer attached to the rear of the semitrailer. A simple, free coupling between the semitrailer and the trailer is not satisfactory because there is no control over the towed trailer and the forward supporting wheels and axle are free to pivot and thereby to cause "jackknifing" and other linkage difficulties that can endanger the entire combination. Rigid connection arrangements between the semitrailer and the trailer are not acceptable because the truck is too long for desired maneuverability. Typical of such structures are U.S. Pat. Nos. 3,004,772; 3,102,738; 3,288,492; and 3,317,219. It was apparent that a second fifth wheel attachment between the semitrailer and the trailer was necessary. It is not feasible to have a fixed extension structure behind a semitrailer because that would interfere with the rear-en loading and unloading of the semitrailer which is important in being able to service loading docks built to load and unload a semitrailer from the rear. Hence, side loading and unloading has not been adopted, which otherwise would permit an extension from the rear to provide a second fifth wheel.
In general the prior art has attempted to solve this problem by providing a dolly, which is a short turn wheeled section carrying a fifth wheel plate on its upper surface and has means for a towing connection to the semitrailer ahead and will be connected to the trailer behind by means of the second fifth wheel. Such an arrangement is shown in U.S. Pat. No. 3,746,369 where the dolly is an independent vehicular section which is attached to the semitrailer when desired, and in U.S. Pat. No. 3,246,912 where the dolly is a part of the load carrying structure of the semitrailer but which can be detached to become the dolly section. In U.S. Pat. No. 3,374,010 there is shown a semitrailer with an extendible chassis to provide a means for coupling a trailer to a semitrailer through a fifth wheel connection. This arrangement, however, will not satisfy highway transportation laws because for normal loading there are not enough wheels to distribute the load properly. A single pair of wheels at the rear of the semitrailer is moved to support the front of the trailer and no wheels are left to support the rear of the semitrailer. Furthermore, in the normal case the semitrailer has two pair of rear wheels (two axles) and this patent does not provide for that arrangement. Generally, it would be expected that each of the semitrailer and the trailer would have a double axle arrangement at the rear of the vehicle.
It is an object of this invention to provide a convertible chassis for a semitrailer that can readily be extended to provide a chassis for both the semitrailer and a trailer towed behind the semitrailer. It is another object of this invention to provide a novel telescoping extendible chassis for attachment of a trailer to a semitrailer with an appropriate number and positioning of load bearing axles and wheels. Still other objects will become apparent from the more detailed description which follows.